The basic objective of this study is to test the hypothesis that the age-related decrease in protein synthesis in rat liver is due to changes in mRNA synthesis or in the stability of mRNA. The synthesis of mRNA will be followed by the incorporation of radioactive precursors into mRNA by isolated liver parenchymal cells from young and old rats. The specific activity of the precursor pool in the isolated liver parenchymal cells will be determined and used to calculate the actual rate of mRNA synthesis. The incorporation of radioactivity into mRNA will be determined by two methods: (1) measuring the incorporation of radioactivity into polysomal material by isolated liver parenchymal cells in the presence of levels of actinomycin D which inhibit rRNA synthesis but not mRNA synthesis, and (2) following the incorporation of radioactivity into poly A containing RNA (mRNA). The poly A containing RNA will be isolated by the specific hybridization of the poly A segment with oligo dT cellulose or poly U sepharose. To determine the effect of aging on mRNA stability, the molecular length and heterogeneity of the poly A segments isolated from mRNA of liver from young and old rats will be compared. Changes in the length or heterogeneity of the poly A segment will be used as an indication of changes in the stability of mRNA.